While an optical fiber having a core portion and a clad layer each made of plastic substance has poorer transmission property than an optical fiber made of glass, it excels the optical fiber of glass in respect that it endures curvatures without breakage to a smaller minimum radius, has greater impact strength, and warrants greater ease of handling. Further, because the plastic optical fiber offers a greater difference in refractive index between the core portion and the clad layer, it possesses high bonding efficiency at connector portions thereof and thus the processing of its terminal portions is simplified. Owing to these advantages, the plastic fiber can be effectively utilized in a data transmission system at a short distance of not more than several tens of meters. However, the plastic fiber provides drawbacks in that it is easily suffered by an organic solvent, and it does not provide thermal resistance at a temperature of about 80.degree. C. or more, so that the plastic fiber undertakes several restrictions in practical use.
Efforts have been made to date, therefore, to enhance durability of the plastic fiber by applying various kinds of coatings on the outer peripheral surface thereof. However, such coatings inadvertantly affect initial characteristics of the plastic fiber. Therefore, it was difficult to enhance durability of the plastic fiber by the coating layer, simultaneously maintaining the inherent properties of the plastic fiber. For example, virtually, conventional coatings for the plastic fibers available on the market are a low-density PE contained carbon (FIGS. 1 and 2). Although such a coated plastic fiber shows slightly better durability against solvents than an uncoated one, the initial characteristic of transmission loss of the coated plastic fiber is worse than that of the uncoated plastic fiber. A plastic fiber coated with PVC (FIG. 3) has been proposed to enhance flame retardancy in addition to the durability against solvents. In this type of coating, however, even though anti-flamability is improved, but such type of plastic fiber is more susceptible to the action of organic or inorganic solvents. It has another problem that a plasticizer in the PVC layer moves into the fiber and deteriorates the properties of the fiber. When PVC capable of preventing such moving of the plasticizer is used, the deterioration due to the plasticizer no longer exists, but the deterioration of the properties still persists due to an external solvent.